The Diet Doomer is a treasure in Pikmin 2 that is actually a well wrapped strawberry truffle. It can be found in the 1st sublevel of the Submerged Castle. It is the 36th treasure on the List of Pikmin 2 treasures.
Captain Olimar's Journal
"My ship can't analyze taste, so I've taken that burden upon myself. It's a rough job, but it's up to the captain to step up and volunteer for the most grueling duties. Mmm... Tastes... Good. I could eat this treat endlessly. I'd better test that hypothesis..."
"Alas, we machines know nothing of eating! The ultimate pleasure... If only I had a mouth!"